The protein fraction of bovine milk is associated with health. The health promoting properties reside not only in the nutritional aspects of this protein fraction but also in the various health promoting factors present.
Milk proteins consists of approximately 80% caseins. The remaining protein is accounted for by a variety of whey proteins. The casein fraction is the main source of amino acids, calcium and phosphate all of which are required for growth of the young animal. The whey protein fraction is also a source of amino acids and in addition it contains several bioactive and putative health promoting proteins such as immunoglobulins, folate binding protein, lactoferrin, lactoperoxidase and lysozyme. It is also known that upon metabolisation of the casein and whey protein fractions a number of new bioactive peptides are formed. Examples of such newly formed bioactive peptides include casomorphins, casokinins, immunoglobulins, immunopeptides, caseinephosphopeptides, lactiphins and lactoferricin. Therefore, the use of casein and whey proteins in the combination in which they occur in milk offers significant nutritional and health benefits.
More recently industrially prepared hydrolysates of milk proteins were also found to contain newly formed bioactive peptides and notably ACE-inhibitors to fight hypertension.
The white appearance of milk is caused by the scattering of light by fat globules and casein micelles. Skimmed milk, i.e. milk from which all fat has been removed, is still white because of these casein micelles.
The whey protein fraction of the milk, i.e. milk after removal of both the fat and casein fraction, is a yellowish but clear protein solution which is rich in various proteins, peptides, lactose, minerals and vitamins. All of these constituents are completely soluble even under acidic conditions. Nevertheless the dissolution of whey proteins may yield turbid solutions as the result of partial denaturation during spray drying. Partial enzymatic hydrolysis can improve the dissolution characteristics of these somewhat denatured spray dried whey proteins. More exhaustive enzymatic hydrolysis of whey proteins further improves their solubility but also leads to modest increases in bitterness and levels of free amino acids present. The usual aim of more exhaustive enzymatic hydrolysis of whey proteins is to achieve a reduction in allergenicity and an improved intestinal uptake. Especially the reduced allergenicity aspect is commercially important. For example in different countries of Northern Europe cow's milk intolerance has been diagnosed in almost 3% of general populations of infants in the first two years of life. Beta-lactoglobulin belongs, together with the caseins to the major allergens in bovine milk. Adults rarely exhibit bovine milk allergies and specialised products for this group must be tailored to be easily assimilable, provide a good taste and exhibit good shelf stabilities, especially under acid conditions. It is therefore not surprising that there exists considerable literature regarding the exhaustive enzymatic digestion of whey hydrolysates aimed at clinical, dietetic and sports applications as well as infant nutrition.
In contrast with whey, casein is rich in hydrophobic amino acids so that its hydrolysates are notoriously bitter and tend to have rancid and brothy off-tastes. Due to their extreme bitterness enzymatically hydrolysed caseins find limited application only. Moreover their high content of hydrophobic amino acids make casein derived peptides difficult to dissolve, especially under acid conditions.
Processes for the preparation of partial casein hydrolysates described in the literature generally involve multi-step hydrolyses with a few endoproteases followed by incubation with one or more exoproteases. Combinations of various endoproteases are commonly used to obtain the high Degree of Hydrolysis (high DH) required to minimise possible allergenic reactions and to improve the solubility. Subsequent incubation with exoproteases releases amino-or carboxyterminal amino acid residues to minimise bitter off-tastes. However, the release of free amino acids implies losses in yield and a diminished nutritional value. Because high levels of free amino acids may also result in brothy off-tastes and increased osmotic values of the final hydrolysate, additional processing steps to remove free amino acids, and strongly hydrophobic peptides which account for the bitter off-tastes, are common practice.
Patent application EP 0 610 411 describes completely soluble casein hydrolysates of good organoleptic quality with low molecular weight peptides and a DH value in the order of magnitude of 15 to 35%.
Patent application WO 96/131744 describes a method for production of a milk protein hydrolysate characterised by a hydrolysis reaction involving any neutral or alkaline protease from Bacillus in combination with an Aspergillus enzyme complex comprising both endo-and exopeptidases and a degree of hydrolysis between 35% and 55%.
Patent application EP 384 303 describes a method for production of a protein hydrolysate exhibiting low bitterness and a low DH value using an aminopeptidase.
Patent application EP 223 560 describes a method for production of milk proteins by means of a sequential enzyme hydrolysis.
Patent application EP 0 631 731 describes a partial hydrolysate of a protein mixture comprising whey protein and casein wherein the hydrolysate has a degree of hydrolysis between 4 and 10% and a low bitter hydrolysate is obtained using a combination of trypsin and chymotrypsin.
Patent U.S. Pat. No. 4,600,588 describes a milk protein hydrolysate consisting of acid precipitated casein that has been treated with a.o. an acid fungal protease.
Patent application JP11243866 describes a casein hydrolysate useful for drinks and food which is tasteless and odorless and has a degree of hydrolysis of 17 to 30%.